High-power Single-mode Operation Research Articles

High-power, stable single-mode CW operation of 1550 nm wavelength InP-based photonic-crystal surface-emitting lasers

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest because of applications that span facial recognition and 3D imaging in mobile telephones, Light Detection and Ranging (LiDAR) for autonomous vehicles, and Pulse Amplitude Modulation (PAM-4) for high-speed (>100 Gbps) data transmission within data centers. A key challenge with oxide-confined VCSELs is that the strong refractive index contrast between the oxidized ~AlAs aperture and surrounding AlGaAs layers combined with the oxide aperture size create a photonic cavity that fundamentally supports multiple transverse modes. When the size of the aperture is reduced to create a single-mode cavity, the output power is limited by the electrically pumped gain region within the aperture. There is thus a fundamental trade-off between size and power which manifests itself in a limited capability to simultaneously achieve high power and single mode operation. Various methods have been employed by groups working on VCSELs to overcome this limitation. Neglecting external cavity methods which are fundamentally not scalable, the most promising results have been achieved by groups using etched surface relief where a pattern is etched in the top layer or layers to change the standing wave pattern in the cavity and reduce modal gain by lowering the magnitude of the electric field in regions overlapping with the quantum wells, which provide laser gain. A downside of this approach is that material is removed, impacting the uniformity of current injection across the area of the device. A key question of interest is therefore how to achieve mode suppression in large area devices in a way that does not impact current injection. In this presentation, novel methods for mode control that rely on spatial variation of modal gain across the area of the VCSEL aperture will be discussed, and results on high-power single-mode operations of VCSELs will be presented. Two distinct mode control methods will be reviewed. The first method of mode control uses deposited optical coatings on the top surface of the VCSEL to spatially modify the electric field standing wave in the quantum well heterostructure region. Through patterning the coating in either a pillar or anulus, specific modes can be suppressed by selectively changing the gain available for those modes. These filters have been made using both SiO2/TiO2 stacks and alternatively a single layer of silicon. The capability to pattern these layers in a wafer-scale process makes this method attractive for volume manufacturing. The second method of mode control utilizes impurity-induced layer disordering via patterned diffusion of Zn to selectively intermix Distributed Bragg Reflector (DBR) layers, thereby creating a top laser mirror whose reflectivity varies spatially across the aperture. Designing the diffused region to maximize overlap with high-order modes while minimizing overlap with the fundamental mode provides mode suppression and enhanced single-mode power. An added benefit of this approach is a reduction in resistance through the device top DBR layers brought about by the higher p-doping level in the Zn-diffused regions, minimizing Ohmic losses and heat generation. Through the use of strain control in the diffusion mask, diffusion front shape is also shown to be controllable in a way that can be used to minimize the impact of the diffused region on fundamental mode reflectivity. Tensile and compressive strain of diffusion masks are shown to have differing effects on vertical (into the device) versus lateral (across the area of the device under the mask) diffusion, allowing more precise spatial control of reflectivity. Using these methods, single-mode output powers in excess of 10 mW have been demonstrated, with side-mode suppression ratios in excess of 30 dB. These results will be discussed, as well as further work to enhance VCSEL performance. Figure 1

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest due to performance needs driven by facial recognition and 3D imaging in mobile telephones. Two distinct mode control methods that rely on exploiting the spatial field distribution of optical transverse modes to achieve high-power single-mode operation in VCSELs will be discussed. The first method uses a surface-deposited optical coating of multilayer SiO2/TiO2 or single-layer silicon to achieve single-mode emission. The capability to pattern these layers in a wafer-scale process makes this method attractive for high-volume manufacturing. The second method utilizes impurity-induced layer disordering (IID) to selectively intermix the top distributed Bragg reflector (DBR) in a VCSEL, thereby creating a mirror whose reflectivity spatially varies across the aperture. Using these techniques, single-mode output power in excess of 10 mW has been demonstrated with side-mode suppression ratios in excess of 35 db.

Confined-doped active fiber enabled all-fiber high-power single-frequency laser.

In this paper, we investigate the performances of an in-house fabricated confined-doped active fiber in the applications of all-fiber high-power single-frequency amplifiers. A 210-W single-frequency single-mode fiber laser is obtained directly, which confirms the excellent performance of the confined-doped active fiber for high-power single-mode operation. To further demonstrate the power scalability of the fiber amplifier, the strategy of applying a temperature gradient along the active fiber is investigated numerically and experimentally, and an up to ∼75% enhancement of the stimulated Brillouin scattering (SBS) threshold is achieved. As a result, a 368-W single-frequency fiber laser is obtained with the beam quality factor of Mx2 = 1.19, My2 = 1.26. Overall, the technique of the confined-doped active fiber provides a promising approach to scale the output power of single-frequency single-mode fiber lasers.

High-power single-mode vertical-cavity surface-emitting lasers using strain-controlled disorder-defined apertures

Strain-engineered diffusion masks deposited via plasma-enhanced chemical vapor deposition are demonstrated to control the curvature of the zinc diffusion front and, hence, disordering front, in disorder-defined vertical-cavity surface-emitting lasers (VCSELs) for enhanced high-power single-mode operation. Tensilely strained silicon nitride diffusion masks are applied to limit the lateral undercut of the disordering front, thereby minimizing the interaction between the disordered region of the distributed Bragg reflector and the fundamental mode. This results in higher threshold modal gain and absorption losses from the disordered region for higher-order modes while enabling greater output powers for fundamental-mode operation in single-mode impurity-induced disordered VCSEL designs. Using this technique, 850 nm AlGaAs VCSELs are shown to operate in a single fundamental mode with record optical output powers in excess of 10 mW and side-mode suppression ratios greater than 35 dB. Electrical and optical performances of these devices are presented in addition to near-field images confirming single-fundamental-mode lasing.

High power continuous wave operation of single mode quantum cascade lasers up to 5 W spanning λ∼3.8-8.3 µm.

In this work, we report high power continuous wave room-temperature operation single mode quantum cascade lasers in the mid-infrared spectral range from 3.8 to 8.3 µm. Single mode robustness and dynamic range are enhanced by optimizing the distributed feedback grating coupling design and the facet coatings. High power single mode operation is secured by circumventing the over-coupling issue and spatial hole burning effect. Maximum single-facet continuous-wave output power of 5.1 W and wall plug efficiency of 16.6% is achieved at room temperature. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field with negligible beam steering is observed. The significantly increased power for single mode emission will boost the QCL applications in long-range free-space communication and remote sensing of hazardous chemicals.

Large mode area inverse index fiber with a graded index profile for high power single mode operation.

We propose a new gain fiber structure with an inverse index profile for high power amplification of a Gaussian single mode beam. A large mode area (LMA) design can be fulfilled with the inverse index profile by implementing a graded index in the depressed core. We numerically show that the proposed gain fiber can guide a single mode Gaussian beam with a large beam area and amplify the beam to a kW level output power.

Intra-cavity patterning - new method of single mode emission enhancement

We demonstrate the results of numerical simulations of 1.3μm InAlGaAs/InP VCSEL with a ring pattern etched at the interface between the cavity and top DBR. We show that the proposed design offers selective confinement of the fundamental mode and strong discrimination of higher order modes. Consequently, VCSEL with ring confinement is capable to operate in a broad range of injected currents in the single mode regime, which facilitates the improvement of maximal emitted power and increase of the wavelength tuning range with very narrow spectral characteristics. High power, single-mode emitting vertical-cavity surface-emitting lasers (VCSELs) are of great interest, since they can serve as high-power, smoothly tuned with current and temperature, coherent-optical sources. Emerging applications include inexpensive and portable gas sensors, telecommunication emitters, and so on. For these applications, it is preferable to have a low-threshold, high side-mode suppression ratio (SMSR) light source. VCSELs inherently emit in a single longitudinal mode; controlling the lateral mode is far more complicated. It can be achieved using a well-established wet oxidation technology, which makes possible single-mode operation only for relatively narrow apertures (providing less than 5 mW of output power) (1). Other methods provide up to twofold higher emitted power but exploit more challenging technologies (2-5). The limitation of high power single mode operation comes from small optical apertures or multimode operation under high injection levels. This paper provides numerical analysis of the ring pattern etched in the cavity of the VCSEL which eliminates higher order mode emission. The ring pattern provides confinement for a fundamental mode and forces higher order modes to be pushed out of the active region. The analyzed structure (Fig. 1a) (6) incorporates InAlGaAs quantum wells within an InP cavity. The cavity is bounded by 35 pairs of Al0.9Ga0.1As/GaAs DBRs from the bottom and 20.5 pairs of Al0.9Ga0.1As/GaAs DBRs from the top. Laterally patterned tunnel junction (TJ) layers are responsible for tunnelling carriers into the

Design and Analysis of New Structure for High-Performance Low-Index Defect-Based Photonic Lattice VCSEL

Single-mode emission from vertical-cavity surface-emitting lasers (VCSELs) with low-index defects is studied numerically. For analyzing optical, thermal, and electrical characteristics of the structures, a comprehensive numerical model is adopted. The nonuniform loss distribution of high-index regions of 2-D square photonic lattice is investigated as a means to improve single-mode behavior of 2-D photonic lattice VCSEL (PhL-VCSEL) with low-index defect. A new structure for higher performance low-index defect-based PhL-VCSEL using a concave configuration of InGaP spacer layer is proposed. Simulation results demonstrates that using the new structure decreases threshold current and the width of the far-field pattern, weakens spatial hole burning effects and increases the modal loss of the competing modes with fundamental defect mode. Therefore, the high-power single-mode operation of the laser is enhanced. Using specific concave configuration considerably increases both threshold current of the first competing side mode and optical output power.

2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers

We demonstrate high power continuous-wave room-temperature operation surface-grating distributed feedback quantum cascade lasers at 4.8 μm. High power single mode operation benefits from a combination of high-reflection and antireflection coatings. Maximum single-facet continuous-wave output power of 2.4 W and peak wall plug efficiency of 10% from one facet is obtained at 298 K. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field without beam steering is observed.

Theoretical investigation of static and dynamic characteristics of vertical-cavity surface-emitting lasers with incorporated two-dimensional photonic crystals

A dynamical model of oxide-confined Vertical-Cavity Surface-Emitting Lasers (VCSELs) with two-dimensional photonic crystals (PCs) incorporated within them so called PC-VCSELs is presented and used to optimise designs for high-power single-mode operation. Three PC-VCSEL designs are considered: (I) with holes in the top DBRs, (II) with PC holes situated between their DBRs and (III) with PC holes etched through the entire VCSEL. A simulated design for a PC-VCSEL of type (I) with holes of d = 2 μm diameter, a = 4 μm lattice constant (d/a = 0.5) and 2.2 μm depth was found to improve the single mode behaviour but not enough to establish single mode behaviour for large apertures. The modulation behaviour was not degraded by the PC. Simulations of type (II) and (III) PC-VCSELs, with the same parameters, have shown multimode operation and degraded modulation properties. Simulations of PC-VCSELs of type (III) with holes of d = 0.2 μm diameter and a = 0.4 μm lattice constant (d/a = 0.5) have shown improved modulation properties and enhanced single mode power for small apertures. In simulation, PC-VCSELs incorporating multiple PC-defects have shown order of magnitude increases in the single mode output power. However, the modulation properties of these VCSELs show degradation due to gain saturation and hopping of the optical modes localized within the PC defects.

Analysis of 2-D Surface-Emitting ROW-DFB Semiconductor Lasers for High-Power Single-Mode Operation

A detailed analysis of 2D surface-emitting active-photonic-crystal (APC) lasers composed of a resonant-optical-waveguide (ROW) array and Pi-phaseshifted second-order distributed feedback/distributed Bragg reflector (DFB/DBR) gratings, made atop the ROW-array elements, is performed. Coupled-mode theory and the transfer-matrix method are used for analysis in the longitudinal direction, and 2D rigorous array modeling is used for analysis in the lateral direction. The gratings, beside their usual functions (i.e., feedback and outcoupling), act as strong lateral-mode (i.e., array-mode) selectors by exploiting the array-modes' different field-intensity overlap with the element regions, at the in-phase-mode resonance. The in-phase array mode is strongly favored to lase at and around its (lateral) resonance due both to better longitudinal-field overlap with the active-grating (i.e., DFB) region <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">and </i> to much lower interelement loss than the other array modes. The loss in the interelement regions that suppresses lasing of the nonresonant modes is due to strong absorption to both the metal contact (i.e., Au) as well as to the semiconductor/metal grating layers (i.e., GaAs-Au) in the element regions. The intermodal discrimination can be further enhanced by introducing strong free-carrier absorption in the interelement regions. For 20-element arrays with 700 mum/600 mum DFB/DBR gratings, emitting at 0.98 mum and of 100-mum-wide lateral dimension, the intermodal discrimination, DeltaJ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> , is high (~500 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at the in-phase-mode resonance, and > 70 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as the regrown interelement-region thickness t is varied (during fabrication) over a 0.02-mum range. The external differential quantum efficiency reaches 58%, and the guided-field intensity profile is virtually uniform in both the lateral and longitudinal directions. For 40-element devices, the projected CW power in a single, diffraction-limited lobe is 2.4 W at 10 x threshold. For 20-element arrays with intentionally introduced free-carrier absorption in the interelement regions, DeltaJ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> is very high (~6500 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at the in-phase-mode resonance, and >70 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as t is varied over a 0.035-mum range. The projected single-mode, diffraction-limited continuous-wave power, from 40-element devices at 10x threshold, is similar: 2.4 W.

High-power single-mode vertical-cavity surface-emitting lasers

This letter reports a design for high-power single-mode operation in vertical-cavity surface-emitting lasers by means of modal gain control using two different sized current apertures to shape the injection-current profile. A smaller current aperture is located several mirror-pairs away from the active region in the p-mirror and a larger current aperture is located in the first n-mirror-pair next to the active region. Both theoretical modeling and experimental test results show substantial improvement in the optical mode behavior using this approach when compared to a traditional single-aperture design. A clear trend of the spectral purity in the modal behavior under continuous wave and pulsed conditions is demonstrated and is in good agreement with theoretical predictions. The best design tested demonstrated a room-temperature continuous wave power output of 7.5mW with a side mode suppression ratio of 20dB.

Formation of vertical ridge structure in 660nm laser diodes for high power single mode operation

We report a method for formation of a ridge with almost vertical sidewalls in the fabrication of ridge stripe AlGaInP laser diode structures, which cannot be easily achieved when a conventional wet-etch process is used, for high power operation with a fundamental transverse mode. By depositing additional oxides protecting layers on the sidewalls after the dry-etch process, a wet-etch process, which is necessary to remove the plasma-induced damaged portion of the etched region, can be safely applied and the vertical ridge shape maintained. Using this method, the ideally rectangular ridges were obtained and the single mode operation of 660nm laser diode with up to 270mW output power was successfully achieved.

Design for high-power single-mode operation from 2-D surface-emitting ROW-DFB lasers

Stable single-mode single-lobe operation to high powers is predicted for two-dimensional surface-emitting lasers, if second-order distributed feedback/distributed Bragg reflector (DFB/DBR) gratings are preferentially placed in the elements of a resonant-optical-waveguide array. Beside their usual functions (i.e., feedback and outcoupling), the gratings act as an effective array-mode selector due to different interaction with the gratings of different array modes. The in-phase array mode is strongly favored to lase around its (lateral) resonance due to better field overlap with DFB region and lower interelement absorption loss than for nonresonant array modes. For 20-element arrays with 700/600 /spl mu/m DFB/DBR gratings, emitting at /spl lambda/=0.98 /spl mu/m, high (/spl sim/100 A/cm/sup 2/) intermodal discrimination /spl Delta/J/sub th/ is obtained. /spl Delta/J/sub th/ is enhanced to /spl sim/225 A/cm/sup 2/ by introducing free-carrier absorption in the array-interelement regions.

High-power single-mode ZnO thin-film random lasers

The possibilities to realize high-power single-mode ultraviolet lasing in highly disordered zinc oxide (ZnO) thin films are investigated. An effective one-dimensional time-domain traveling-wave model is developed to simulate random laser action in ZnO thin-film waveguides with ridge structure. Spectral and spatial redistributions of lasing modes, due to lasing mode localization and repulsion, are shown in highly disordered media. Hence, by controlling the photon density distribution, selective excitation of lasing modes can be achieved. A coupled-cavity ZnO thin-film random laser is also proposed to achieve high-power single-mode operation under nonuniform pumping. Preliminary experimental results have verified the possibility to realize high-power single-mode emission by the use of the proposed coupled-cavity design.

Transient Response of ARROW VCSELs Under External Optical Feedback

The effect of external optical feedback on the transient response of antiresonant reflecting optical waveguide (ARROW) vertical-cavity surface-emitting lasers (VCSELs) is studied. It can be shown that the proper design of ARROW can suppress the excitation of high-order transverse leaky mode as well as increase the critical feedback strength so that stable high-power single-mode operation of VCSELs can be obtained even under the influence of strong external optical feedback.

Single-transverse-mode vertical-cavity lasers under continuous and pulsed operation

Using a hybrid ion implanted/selectively oxidized device structure, we report high-power single-mode operation of an 850-nm vertical-cavity laser. Under continuous-wave operation, >4 mW of single-mode power with 45 dB of side-mode suppression is achieved. The spectral behavior under pulsed modulation is determined to be influenced by thermal lensing. When biased to threshold, single-mode operation with >35-dB side-mode suppression is obtained for large signal modulation.

Modal characteristics of ARROW-type vertical-cavity surface-emitting lasers

Antiresonant reflecting optical waveguide (ARROW-type) vertical-cavity surface-emitting lasers are designed for high-power single-mode operation. A detailed full-vector finite-difference time-domain (FDTD) study shows strong modal discrimination in favor of the fundamental mode for large aperture (>6 /spl mu/m), large index-step (/spl Delta/n>0.025) simplified ARROW-type devices. The FDTD model identifies the polarization-dependent radiation losses of the higher order modes, which prevent them from reaching laser threshold.

Gaussian-like tapered grating quarter wave-shifted DFB semiconductor lasers for high-power single-mode operation

Gaussian-like tapered grating quarter wave-shifted DFB semiconductor lasers for high-power single-mode operation